1. Field of the Invention
This invention relates to the preparation of electrical interconnection substrates, particularly high density multiple interconnect (HDMI) substrates, to receive integrated circuit (IC) chips by both wire bonding and solder connections.
2. Description of the Related Art
HDMI multichip module wafers are typically fabricated to accept integrated circuit (IC) dies by either wire bonding the circuit leads to (typically gold) wire bond contact pads on the wafer, by a tape automated bonding (TAB) process, by solder bonding the leads to solder contact pads on the wafer, or by a combination of both wire and solder bonding. Working with both solder bonds and gold wire bonds on HDMI wafers is more complicated than the wave soldering process commonly used in printed wiring board (PWB) fabrication. With PWBs, as opposed to HDMI wafers, a patterned copper network on the board is coated with solder that wets only on the copper contact pads, not on the epoxy PWB material. There is accordingly no need to mask the PWB during the wave soldering process. With HDMIs, on the other hand, the interconnection substrate cannot simply be subjected to a wave soldering process, since that would preclude gold wire bonding to the same substrate. Even if a photoresist mask were used to protect the contacts intended for wire bonding, such masks are not compatible with the temperatures employed in wave soldering and the hot air knife typically used to remove unwanted solder. Furthermore, gold in the presence of solder forms brittle, undesirable intermetallic compounds.
The typical HDMI is initially provided with gold contact pads only, not solder contact pads or "bumps". The process previously used to allow for soldered contacts as well as wire bonded contacts involves first coating the HDMI wafer with a layer of photoresist, and then forming openings through the photoresist to expose the contact pads that are intended for solder bonding. The gold film is next selectively etched from the exposed contact pads, followed by a solder electroplating step to deposit solder in the contact pad locations from which the gold was previously removed. The wafer is then heated to a temperature at which the lead and tin in the solder melt and form an alloy. The photoresist is removed, along with the overlying unwanted solder, prior to formation of the solder alloy at the desired contact pad locations.
A primary problem with this method is that, because the plated solder must be heated to achieve the proper alloy, the alloy may not be consistent from wafer to wafer; this can lead to variations in solderability, which is an serious drawback. Moreover, the wafer preparation process is complicated by the additional steps required to remove the gold from the desired solder contact pads, and to afterwards heat the deposited solder sufficiently to form an alloy. The heat treatment must be performed in an inert atmosphere, typically nitrogen, which is fairly expensive to provide. The additional processing increases both the cost and the time required to prepare the HDMI substrate.